The invention relates to digital communications systems. More particularly, the invention relates to techniques for estimating parameters that are useful or required for demodulating data that is sent in packets including a header and a payload without requiring that training sequence or pilot symbols be available for the parameter estimation.
For a digital communication system to operate properly, frequency estimation and timing recovery should be performed. In addition, other parameters such as phase and channel quality may be estimated. Frequency estimation is necessary because the crystal oscillators (XOs), used in transmitters and receivers, have some deviations (or spread) from their nominal frequency. For example, if the spread in each one of the XOs (i.e., both the transmitter XO and the receiver XO) is 20 ppm, and the carrier frequency of the system is 2.45 GHz, such as in the Industrial, Scientific and Medical (ISM) radio frequency band that is used for BLUETOOTH™ wireless systems, then the frequency offset may be as much as 98 kHz (i.e., 2·20·10−6·2.45·109 Hz). In most applications this is unacceptable. Therefore, it is necessary to counteract the frequency offset. Conventionally, this is accomplished by estimating the frequency offset and then compensating for it. Clearly, if the frequency offset could be exactly estimated, then the frequency offset could be perfectly compensated for. Thus, the system performance would be the same as if there had been no frequency offset at all. However, a perfect estimate of the frequency offset is not possible because there will be some error in the estimation. After compensating for the estimated frequency offset, there will be some residual frequency error that corresponds to the estimation error. The residual frequency error may be on the order of 5 kHz, which may be acceptable in terms of system performance.
The receiver also has to determine when a packet starts (i.e., frame synchronization) and exactly when to make a decision on the received symbols, referred to as symbol synchronization.
In order to ease both the frequency estimation and the timing recovery, a number of known symbols, sometimes referred to as pilot symbols or a training sequence may be sent. FIG. 1 shows a block diagram of a signal that includes a training sequence 100. The signal also contains a frame synchronization word 102, a header 110 and a payload 120. Although the training sequence 100 is shown at the start of the signal, it may be located at other positions within the signal, such as the center of the signal. Additionally, the locations of the frame synchronization word 102, header 110 and payload 120 are not restricted to the specific locations shown. Knowledge of the training sequence 100 facilitates the receivers estimating the above-described parameters. The frame synchronization word 102 is typically significantly shorter (e.g., 16 binary symbols) than either the header 110 (e.g., 112 binary symbols) or the training sequence 100. The frame synchronization word 102 is detected during signal reception to facilitate locating the first symbol of the header 110. Although the frame synchronization word 102 contains known symbols, the number of symbols is insufficient to aid in time and frequency estimation in a data aided manner.
Alternatively, the frequency estimation and the symbol synchronization may be performed on the data directly. Thus, no symbols are wasted because pilot symbols are not used (i.e., the frequency spectrum is more effectively used). FIG. 2 shows a block diagram of a signal that does not include a training sequence or pilot symbols. The signal contains a frame synchronization word 202, a header 210 and a payload 220. As noted above, the locations of the frame synchronization word 202, header 210 and payload 220 are not restricted to the specific locations shown. However, if no pilot symbols are used, then the estimation typically will be worse than estimation that uses pilot symbols. Therefore, the system performance in terms of bit or packet error rate will be worse. Once again, the number of symbols in the frame synchronization word 202 is insufficient to aid in time and frequency estimation in a data aided manner.
In digital communication systems in general, and in wireless communication systems in particular, data is often sent in a packet having a frame synchronization word, a header and a payload. The header contains vital information for the link, such as control signals, the length of the packet and the like. If the header is not received correctly, the entire packet is disregarded and a retransmission of the entire packet is requested. The payload contains the actual user information to be transmitted. If the payload is received in error, a retransmission of that particular payload is requested. However, if the header is correct, the information in the header may be used regardless of errors in the payload. Therefore, an incorrect header is worse than an incorrect payload. Thus, the header is typically better protected, either by using a more powerful Forward Error Correction (FEC) code, or by employing a more robust modulation format. For example, in a BLUETOOTH™ wireless system, the header is always protected by a (3,1) repetition code, whereas the payload might be un-coded.
Since the header is more robust, the accuracy requirement for estimating parameters of the header is typically not as severe as it is for the payload. Therefore, the header often will be correctly demodulated even though parameters, such as frequency offset, timing, and the like, are only coarsely estimated. If pilot symbols are not used, then the header symbols may be used for performing the estimation. Typically, the estimation may be performed by two standard techniques either by using non-data aided estimation or by using decision directed estimation.
In non-data aided estimation, the information carrying part is simply removed. For instance, in the case of M-ary phase shift keying (PSK), this is achieved by multiplying the phase by M. However, this multiplication also enhances the noise. Therefore, the estimator performs worse than a data-aided estimator.
In decision directed estimation, the estimation is done in a decision feedback fashion. First a decision is made, then this decision, which is assumed to be correct, is used to perform the parameter estimation. If the decision is correct, then the parameter estimation will be as good as if the estimation had been data aided. However, the feedback sometimes is erroneous, which will quite drastically degrade the performance. Consequently, if the channel conditions are good (i.e., if the probability for erroneous feedback is very small), then decision directed parameter estimation is feasible. However, if the channel conditions become worse, then an estimator based on the principle of decision directed parameter estimation is not suitable. Since channel conditions often change, particularly in wireless communications, the channel quality cannot be guaranteed to be good.
The prior art systems rely on training sequences or pilot symbols to provide accurate parameter estimation. However, not all systems use training sequences or pilot symbols. Additionally, if the signal has a training sequence or pilot symbols, then the signal has less capacity for the payload. Therefore, a method and apparatus that can provide accurate parameter estimation without training sequences or pilot symbols are needed.